Blue Moon over North America

May 30, 2007: At 9:04 pm Eastern Daylight Time on May 31st, the full moon over North America will turn blue.

Not really. But it will be the second full moon of May and, according to folklore, that makes it a Blue Moon.

Above: The first full moon of May 2007, photographed May 2nd by Tony Wilder of Wisconsin. May's second full Moon on May 31st will probably look as gray as this one, although according to folklore, it is "blue."

If you told a person in Shakespeare's day that something happens "once in a Blue Moon" they would attach no astronomical meaning to the statement. Blue moon simply meant rare or absurd, like making a date for "the Twelfth of Never."

But "meaning is a slippery substance," writes Philip Hiscock of the Dept. of Folklore, Memorial University of Newfoundland. "The phrase 'Blue Moon' has been around a long time, well over 400 years, and during that time its meaning has shifted."

The modern definition sprang up in the 1940s. In those days the Maine Farmer's Almanac offered a definition of Blue Moon so convoluted even professional astronomers struggled to understand it. It involved factors such as ecclesiastical dates of Easter and Lent, tropical years, and the timing of seasons according to the dynamical mean sun. Aiming to explain blue moons to the layman, Sky & Telescope published an article in 1946 entitled "Once in a Blue Moon." The author James Hugh Pruett (1886-1955) cited the 1937 Maine almanac and opined that the "second [full moon] in a month, so I interpret it, is called Blue Moon."

This was not correct, but at least it could be understood. And thus the modern Blue Moon was born. A detailed account of the story may be found here.

Surveying the last four centuries of literature and folklore, "I have counted six different meanings which have been carried by the term," recounts Hiscock. In song, for instance, Blue Moons are a symbol of loneliness; when love conquers all, the Blue Moon turns gold. (See old Elvis records for more information.) "This makes discussion of the term a little complicated," he says.

One complication is that the Moon can turn genuinely blue, as shown in this photo taken by Tom King of Watauga, Texas:

Above: A blue Moon photographed in October 2003 by Tom King of Watauga, Texas. "Here is

from a different night provided as a sanity check to assure you that my moon shots do not always have this blue hue," says King.

"I had never paid any real attention to the term 'Blue Moon' until one October evening in 2003," he recalls. "I had my telescope set up in the backyard and the moon began rising in the east with a strange blue tint I had not seen before."

The cause of the blue was probably tiny droplets of water in the air. "The air was damp and heavy with moisture," notes King. When water droplets are about 1 micron (one millionth of a meter) in diameter, they strongly scatter red and green light while allowing other colors to pass. A white moonbeam passing through such a misty cloud turns blue.

Clouds of ice crystals, fine-grained sand, volcanic ash or smoke from forest fires can have the same effect. "The key," notes atmospheric optics expert Les Cowley, "is that the airborne particles should all be of very similar size, a micron or so in diameter." Only then do they scatter the correct wavelengths of moonlight and act as a blue filter.

There are other reasons for blue Moons, he notes. "Our eyes have automatic 'white balances' just like digital cameras. Go outdoors from a cozy cabin lit by an oil lamp (yellow light) and the Moon will appear blue until your eyes adjust."

What kind of Blue Moon will you see this week? There's only one way to find out!

What about Europe? Because of time zones, this week's full Moon occurs over Europe on June 1st rather than May 31st. In Europe, therefore, it is the month of June which has two full Moons, and a Blue Moon on June 30th. In North America, May has two full Moons and a Blue moon on May 31st.

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A note of irony: Blue moons that really exist are wildly unpredictable. Who can say when a volcano will spew perfectly-proportioned ash into the stratosphere? Or when a Texas mist of micron-sized water droplets will waft in front of the rising moon?

The absurd blue moons of folklore, on the other hand, no longer take anyone by surprise. Full moons are separated by 29.5 day intervals--the moon's synodic period. By counting out synodic intervals, and checking to see when a pair of full moons falls into a single 30- or 31-day month, we can easily and precisely figure out when the next blue moon is due: Dec. 31, 2009. Mark your calendar.

"This new blue moon has a kind of technical meaning which most of the earlier ones lacked," notes Hiscock. "Perhaps as a result it will last a whole lot longer."

Folklore of the Blue Moon -- by Philip Hiscock of the Dept. of Folklore, Memorial University of Newfoundland

Blue Moons and Lavender Suns -- (Alaska Science Forum)

What's a Blue Moon? According to the editors of Sky & Telescope, the definition of "blue moon" as the second full Moon in a month is a mistake.